This invention relates to a method for the determination of the form of graphite in spheroidal and compacted/vermicular (CV) graphite cast irons.
The form of graphite in cast irons is depend on the conditions of solidification. Accordingly, it is preferable to determine the form of graphite before casting.
For most of these cast irons, graphite exists in the form of flake (similar to corn flake), snowflake, spike, C/V and the like. It is, however, sufficient to know one of the forms of graphite, or the portion of spheroidal graphite and C/V graphite mixed in cast irons.
In order to find out the form of graphite in cast irons, usually, there are three methods such as, (1) a method of using an electron microscope, in which a portion of molten cast iron has been collected as a sample, and after solidification of the sample, the surface of the sample is polished and inspected by an electron microscope, (2) a method of measuring velocity of ultrasonic wave passing through the sample, and (3) a method of inspecting eutectic temperature of molten cast irons.
In these methods, the method of using the microscope and ultrasonic waves take much times for investigation and leads to noticeable errors. Accordingly, only the method of the thermal analyzing is practical and useful.
However, in the thermal analyzing method, it is necessary to convert graphite in molten cast iron into spherodized graphite.
In the case of graphite in molten cast iron which has not been converted into spherodized graphite, it is extremely difficult to determine the form of crystallized graphite, even if said molten cast iron is thermally analyzed.
The addition of spheroidizing agent to C/V graphite requires strict control of molten cast irons.
In the light of these problems, it is a principal object of the invention to provide a novel method of the determination of the form of graphite existing in spheroidal graphite cast iron before crystalizing.
It is a further object of the invention to provide a method for the determination of the form of graphite by measuring the quantity of dissolved oxygen containing in molten cast iron and eutectic temperature thereof.
It is a still further object of the invention to a method for estimating the proportion of spheroidal graphite and C/V graphite containing in the molten cast irons by thermal analyzing.
According to the invention, for the purpose of attaining the above objects, in the first step, a portion of the molten cast iron is collected in a sample testing vessel, and then quantity of oxygen dissolved in the molten cast iron by using a conventional oxygen sensor.
From the behavior of oxygen dissolved in molten cast iron, the effectiveness or ineffectiveness of spheroidizing or vermiculating may be decided. When the effect is recognized, eutectic temperature of the molten cast iron can be measured and it is possible to decide the form of graphite in the molten cast iron and the proportion of spheroidal and C/V graphite existing therein.
If the above effect has not been recognized, it is decided that graphite exists in the form of flake.
The quantity of dissolved oxygen depends upon the temperature thereof, and the quantity of dissolved oxygen therein may be measured by using an oxygen sensor is mainly influenced very much by silicon content in the sample.
Accordingly, it is impossible to decide the effect of spheroidizing of the graphite in the molten cast iron from the above quantity of dissolved oxygen may not be decided, and therefore, it is preferable to obtain the relation between the quantity of dissolved oxygen and spheroidizing or compacted/vermiculation.
If, for example, the quantity of dissolved oxygen in the molten cast iron is less than 1 ppm, the effect of spheroidizing or c/v treating will be recognized, but if the quantity of dissolved oxygen is more than 2 ppm, graphite in the molten cast iron will exist in the form of flakes (A or D type), from which it is considered that the effect of C/V treating is not shown.
Even though the chemical composition of flake graphite cast iron (A-type) is identical to one of D-type, there are many differences between the two in ability of forming eutectic graphite nucleus. If a large number of nucleus are existed in eutectic graphite, these nucleus will form flake graphite cast iron (A-type), but if small number of nucleus are existed therein, they will form flake graphite cast iron (D-type), because eutectic austenite in molten cast iron is solidified in the first place.
If eutectic temperature of molten cast iron is higher than about 1141xc2x0 C. at which spheroidized graphite phase is changed to C/V graphite phase (this temperature is refereed to as xe2x80x9ca threshold temperaturexe2x80x9d), molten cast iron contains a large amount of nucleus to form spheroidal graphite, but if it is lower than the threshold temperature, it contains a small amount of nucleus.
Accordingly, the eutectic temperature of molten cast iron in each of the above samples may be measured by using a cup like sampling vessel. Then it is able to decide that if the eutectic temperature of the sample is higher than the threshold temperature, it forms spherodized graphite cast iron, but if it is lower than the threshold temperature, it forms C/V graphite cast iron.